Various kinds of couplings are used in a steering apparatus of a vehicle, including an elastic shaft coupling, which comprises an elastic member of rubber interposed between, for example, a yoke of a universal joint and a shaft to be inserted in this yoke, thereby absorbing vibration of an engine or a wheel to suppress transmission of the vibration to a steering wheel.
In such an elastic shaft coupling, a torque is transmitted from the shaft to the yoke through the elastic member in a range having a low torque. However, in a range having a high torque, the transmitting is performed upon direct contact of a stopper face which is provided between the yoke and the shaft.
As shown in FIG. 7A, a stopper plate 3 formed with a cut-away portion 2 and a disc-shaped elastic member 4m are secured to a steering shaft 1, and a flange 5m of an unrepresented yoke is attached to the shaft 1 through a bush 6.
A pin 8m is inserted through the elastic member 4m and the flange 5m of the yoke through a washer 7m. An end of the pin 8m on the yoke side is a male thread shaft which is thread-engaged with a nut 9 to be fixed. The other end of the pin 8m is to be brought into contact (engagement) with the cut-away portion of the stopper plate 3.
Accordingly, in the range with low torque, the elastic member 4m transmits, while being elastically deformed, a steering torque by its deforming resistance. On the other hand, in the range with high torque, the other end of the pin 8m transmits, while being brought into contact (engagement) with the cut-away portion 2 of the stopper plate 3, a steering torque directly to the shaft 1 from the flange 5m of the yoke.
Also in case of FIG. 7B, the structure for transmitting the steering torque is the same as that in the above-described case. However, the end of the pin 8n on the yoke side in this case is fixed to the flange 5n by clinching. In this case also, the pin 8n has a structure with two steps in which it has a large diameter in its part fitted to the elastic member 4n and a small diameter in its part fitted to the flange 5n. 
Also in case of FIG. 7C, the structure for transmitting the steering torque is the same as that in the above-described case. In this case, however, the elastic member 4p is integrally formed with the washer 7p and the end of the pin 8p on the yoke side is fixed to the flange 5p by clinching. In this case also, the pin 8p has a structure with two steps in which it has a large diameter in its part fitted to the elastic member 4p and a small diameter in its part fitted to the flange 5p. 
Though not particularly illustrated, there may another structure for fixing the pin (8m, 8n, 8p) to the flange (5m, 5n, 5p) in which an unstepped pin is fixed by clinching or a stepped pin is fixed by a thread engaging means.
Incidentally, there is a demand for reinforcement of the strength of a fixing structure between the pin (8m, 8n, 8p) and the flange (5m, 5n, 5p).
In such a case, the diameter of the pin (8m, 8n, 8p) may be enlarged. In this case, however, there arise problems such that the weight thereof becomes greater and that the diameter can not be enlarged when the pin interferes with peripheral components. Also in this case, the width of the cut-away portion 2 of the stopper plate 3 has to be increased, which results in an increased size of the whole structure in order to maintain the strength. Further, the diameter of a pin insertion hole of the elastic member (4m, 4n, 4p) has to be enlarged, which may resultantly lead to an enlarged size of the elastic member. Still further, the hole diameter of the flange (5m, 5n, 5p) of the yoke has to be enlarged, which may resultantly lead to an enlarged size of the flange (5m, 5n, 5p). When there is no enlargement of the size or the reinforcement, the strength is deteriorated.
Still, there may be a case that the hardness of the pin (8m, 8n, 8p) is reinforced. In such a case, however, a clinching work may become difficult to perform, or the structure may be easily affected by an impact load, or the manufacturing cost may be widely increased due to an additional thermal processing step of the materials.